The present invention relates to an electrostatic filter suitable for a respirator or the like.
So-called resin wool is a well known filter media, which comprises a fibrous sheet-like component composed of felt or nonwoven fabric made of an animal wool fiber and a resinous component composed of a synthetic resin such as phenolic resin, with which the fibrous component is impregnated or which is coated to the fibrous component. The resin wool media undergoes a friction treatment to generate electrostatic charges thereon. The charged resin wool media has conventionally been used as a high performance electrostatic filter for a respirator such as a dust respirator in view of the fact that the filter provides a high collecting efficiency for fine dust without significantly increasing resistance against breathed air.
Such a well known electrostatic resin wool filter of prior art comprising animal wool felt impregnated with phenolic resin (hereinafter referred to MK resin) is schematically illustrated by FIG. 12 of the accompanying drawings. This filter is based on such a principle that an animal wool fiber develops a positive charge and a phenolic resin develops a compensating negative charge and the filter as a whole is left in an electrostatically neutral condition. Particles such as dust particles are usually positively or negatively charged and attracted and collected by the oppositely charged fiber and resin of the electrostatic filter as these particles pass through the filter. Examples of the electrostatic filter based on such a principle are disclosed by Japanese Patent Publication No. S59-32166 (Patent Registration No. 1256142) entitled "Electrostatic Air Filter" and Japanese Patent Publication No. H03-31483 (Patent Registration No. 1676882) entitled "Electrostatic Air Filter", both of which disclosures are incorporated herein by reference. According to Japanese Patent Publication No. H03-31483 an electrostatic filter comprises a fibrous component made of wool and synthetic polyester (PET) fibers, and a resinous component constituted by a phenolic resin distributed over the fibrous component.
While well known electrostatic filters as have been described above certainly have a good collecting efficiency for solid particles, it has recently been reported by several articles (e.g. Mark W. Ackley: "Degradation of Electrostatic Filters at Elevated Temperature and Humidity", Filtration & Separation, July/August, 1985, 239-242) that these electrostatic filters exhibit a degradation of an efficiency in aerosol collection.
The NIOSH Standard of the U.S. for respiratory devices revised in October, 1995 (42 CFR Part 84 Respiratory Protective Devices, in which a dust respirator is included) requires an efficiency test using aerosol such as oil mist. According to the Standard, the test should be conducted with a particle concentration of DOP aerosol not exceeding 200 mg/m.sup.3, an average particle dia. of DOP being approximately 0.2 .mu.m (0.17-0.22, a geometric standard deviation of .sigma.g&lt;1. 6) and an aerosol flow rate of 85.+-.2 liter/min. The Standard requires that a mask should maintain its collecting efficiency of 95% or higher even after DOP has been continuously supplied by 200.+-.5 mg and the mask to be tested should be preconditioned at a temperature of 38.+-.2.5.degree. C. and a relative humidity of 85.+-.5% for 25.+-.1 hours.
The testing conditions of the NIOSH Standard significantly differ from the testing conditions imposed by the current Japanese National Standard for certifying dust respirators and the Japanese Industrial Standard for testing dust respirators (in both of the Standards quartz particle dust having a particle dia. not exceeding 2 .mu.m is supplied through a mask at a flow rate of 30 liter/min and an increase in the inspiratory resistance is determined as of a moment at which an accumulation of the dust reaches 100 mg.). Probably no respiratory masks equipped with electrostatic filters are commercially available in Japan which meet the requirements of the revised NIOSH Standard. According to the NIOSH test conducted by the applicant of the present invention, the above-mentioned electrostatic filters of the prior art showed that the filters could not meet the reference value required by the NIOSH Standard, as will be described later in reference with FIG. 5.
Generally, in an electrostatically charged substance, static charges are progressively neutralized or discharged if the substance is left as it is for a long time in an environment at elevated temperature and humidity. It is well known that such a progressive decrease of the charges in an electrostatic filter results in degradation in a dust collecting efficiency of the filter. In addition, DOP aerosol particles used in the NIOSH efficiency test are oily liquid of a relatively low viscosity and tend to wet and cover charged surfaces of the fiber as well as of the resin of the filter with oil as the DOP particles accumulate on the filter. The collecting efficiency of the filter is further degraded at an accelerated speed probably because the DOP aerosol accumulated on the surfaces begins to progress through the filter. In order to obtain a respirator filter which meets the NIOSH Standard, therefore, it is essential to provide an electrostatic filter of the prior art with effective countermeasures against the above-mentioned degradation caused by the preconditioning at the elevated temperature and humidity and the DOP aerosol applied by the NIOSH test.
While a phenolic resin used in an electrostatic filter of the prior art is satisfactory so far as its electrostatic characteristic is concerned, a serious problem is caused by its affinity to oily substances such as DOP. More specifically, DOP aerosol develops over the surface of the filter and then penetrate the electrostatically charged resin, with a result that the charges which have been held within the resin may be neutralized or discharged and thereby the collecting efficiency of the filter may be degraded. Accordingly, it can be expected to minimize such a degradation of the collecting efficiency, if it is possible to improve a behavior of the resin against the DOP aerosol without deterioration of its electrostatic feature.